Definition
A decompiler reads a compiled file and reconstructs higher-level code. IL keeps types, names, and method structure, so tools such as ILSpy and dnSpy rebuild C# that looks close to the original. For native code from IL2CPP there is no direct C# decompiler. Instead, a dumper reads global-metadata.dat and a native disassembler shows the machine code with the recovered names attached.
Relevance
A decompiler is how you read an assembly when you want the structure back as C#. From a managed assembly it recovers class names, method logic, and the strings inside them, which is why IL is so much easier to follow than raw machine code.
Example
You open a package DLL in a decompiler to see the public methods the compiler kept. The tool shows the class, the method names, and the logic of each method, close to the C# that was compiled.